1. Field of the Invention
The present invention relates to a drill including a cutting edge a point angle of which varies depending on a position.
2. Description of the Related Art
Examples of the drill including a cutting edge a point angle of which varies depending on a position may be double angle drills disclosed in Japanese Unexamined Patent Application Publication No. 1988-306812 (hereinafter, referred to as document '812) and in Japanese Unexamined Utility Model Application Publication No. 1994-75612 (hereinafter, referred to as document '612).
Such a double angle drill includes a primary cutting edge and a secondary cutting edge. The tip section of the primary cutting edge has a ridgeline shape of a drill for metal. The secondary cutting edge continuously extending from the primary cutting edge is flat and has a smaller point angle than the primary cutting edge.
Any of documents '812 and '612 teaches that the double angle drill is suitable for drilling a hole in a fiber reinforced plastic composite material and a metal material at a time.
With the double angle drill, the primary cutting edge drills a primary hole with a relatively small diameter, and then the secondary cutting edge cuts the outer periphery of the primary hole to drill a secondary hole with a target diameter. During drilling, transient delamination may appear at the periphery of the primary hole of the composite material. The delamination is removed by cutting the primary hole by the secondary cutting edge.
Unfortunately, as mentioned in document '612, the double angle drill typically has low wear resistance. Document '612 discloses a configuration to increase the wear resistance of the cutting edge by covering the cutting edge with a diamond film with a suitable film thickness.
However, the double angle drill has corners at the boundary between the primary and secondary cutting edges and at the outermost periphery of the secondary cutting edge. A stress is likely concentrated at the corners, and hence chipping likely appears at the corners. Thus, such a shape of the drill decreases the wear resistance of the drill.
In order to increase the wear resistance of the cutting edge, the applicant of this application has filed an application for a drill in Japanese Unexamined Patent Application Publication No. 2008-36759 (hereinafter, referred to as document '759). The drill includes a cutting edge. The cutting edge has a point angle which is continuously decreased from a center-position point angle A (herein, 0°<A<180°) at a center position to a maximum-diameter-position point angle of 0° at a maximum diameter position. The cutting edge has a relief angle which is continuously decreased from the center position to the maximum diameter position.
Document '759 describes the point angle of the cutting edge as follows. “The point angle of the cutting edge is decreased from the center-position point angle A to the maximum-diameter-position point angle of 0° in a continuously changing manner. Accordingly, the ridgeline of the cutting edge defines a smooth curve without a corner (point of discontinuity). Also, it is assumed that the curve defined by the ridgeline of the cutting edge is a curve protruding to the outside without a point of inflection. For example, in the ridgeline of the cutting edge, the curve from the center-position point angle A to the maximum-diameter-position point angle of 0° is defined as a part of a parabola. The ridgeline of the cutting edge may partly have a straight line. In this case, the point angle of the cutting edge is continuously changed even at a transition portion between the straight line and the curve (not having a straight line), thereby eliminating a corner.” Also, document '759 describes that the ridgeline of the cutting edge may be an arcuate ridgeline formed of a single arc, instead of the parabolic ridgeline.
With the drill disclosed in document '759, the point angle of the cutting edge is continuously decreased from the center position to the maximum diameter position. Thus, the ridgeline of the cutting edge has no corner, which is likely chipped, and the wear resistance of the cutting edge is increased.
The drill disclosed in document '759 has a cutting edge section mainly for drilling, which is located close to the tip end among the entire cutting edge, and a cutting edge section mainly for reaming, which continuously extends from the former cutting edge and is located close to the rear end. The cutting edge mainly for drilling gradually shifts to the cutting edge mainly for reaming.
The present invention is made as a result of advanced studies on the drill of such a type disclosed in document '759 by the inventors of this application, for a fiber reinforced plastic composite material. The present invention provides a drill that is ideally suited for drilling a hole in the fiber reinforced plastic composite material.
A first object of the studies by the inventors of this application has been to increase reaming performance by a cutting edge section located close to the rear end.
A second object of the studies by the inventors of this application has been to increase cutting performance by a cutting edge section located close to the tip end.